4-20mA variable output

Alan Krich

Other Parts Discussed in Thread: XTR110, XTR116, XTR115

Hi!

I am looking for a circuit which will make give me a 4-20mA (or 0-20mA) variable output. Preferably the input shall be a resistance pot. Use of standard TI products with least number of external components would be great. I am not a professional electronics person and hence the simpler the circuit, better it is!

** IF ABOVE IS NOT POSSIBLE **

Alternatively, I have a Belimo SGF24 ( http://www.belimo.ch/pdf/e/2_Z_4_e5.pdf ) which provides me with a variable 0-10V signal. I "think" using the XTR110 wil do the job. However a bit confused on the layout. Again, a wiring diagram shall be great!

Ta!

Alaln

over 14 years ago

0 Ian Williams over 14 years ago

TI__Mastermind 27715 points

Hello Alan,

Thank you for your question, and welcome to the TI E2E forums!

The simplest way to achieve 4-20mA out with a resistance pot at the input would be by using the XTR115 or XTR116, since both parts have an internal voltage reference that could be used with the appropriate resistors to easily generate an input current. The devices have a current gain of 100, so for 4-20mA out you simply need to apply 40-200uA in. The device is powered by the loop voltage of 7.5-36V (24V typical), and the only other components needed are a bipolar transistor, a decoupling capacitor, two resistors and the potentiometer.

I've provided a .zip file that contains resistor value calculations, a schematic, and a simulation showing the input current circuit. In my example I selected the XTR116 (which has a V_ref of 4.096V) and a 0-100k potentiometer, but if you preferred a different value then of course your numbers would change.

Can you tell me a little about your application? Depending on how you're using the XTR my recommendation may need to be tweaked.

Best regards,

Ian Williams
Linear Applications Engineer
High Performance Linear

XTR116 R-to-I converter.zip

0 Alan Krich over 14 years ago in reply to Ian Williams

Prodigy 60 points

Hello Ian,

Thanks for the detailed reply. Just what I was looking for. I wanted to have a simple variable 4-20mA signal to simulate pressure signal for bench testing . (The controller works off a pressure transmitter)

The XTR116 is locally available and shall build a circuit around it. Would it be possible for you to inform the following. Would be great if you could modify your schematic.

1. Would be preferable to use the a 0—10KΩ potentiometer.

2. Could you please recommend a model / part number for the transistor?

3. Is it possible to integrate a display (amp meter ~ analogue or digital) into the circuit?

Thanks for your help.

Rgds

Alan

0 Ian Williams over 14 years ago in reply to Alan Krich

TI__Mastermind 27715 points

Alan,

Using a 0-10k pot would require a negative R_sh, which of course does not exist in the real world. I've attached another calculations page showing this. Is there a reason you prefer to use 10k? Does the pressure sensor you plan on using have an output resistance in that range?
We recommend three transistors in the data sheet: 2N4922 (TO-225 pkg.), TIP29C (TO-220 pkg.), TIP31B (TO-220 pkg.).
You can easily integrate an ammeter into the circuit. Simply place the meter in series with the I_out pin (pin 4) and you should be good to go. I've added an ammeter symbol to the schematic for your reference.

Best regards,
Ian Williams

XTR116 R-to-I converter (10k pot).zip